1. Field of the Invention
The present invention relates to an exhaust pipe structure for an internal combustion engine. More particularly, to an exhaust pipe structure that focuses attention on an exhaust sound from an internal combustion engine mounted in a saddle-ride type vehicle or the like.
2. Description of Background Art
In a saddle-ride type vehicle disclosed in JP Patent No. 4653064, in the case of a vertical V-shaped four-cylinder internal combustion engine, two exhaust pipes extending rearwardly from two cylinders in one bank of right and left banks are configured to be coupled to a muffler after being collected to a collecting pipe on a rear side. Two exhaust pipes extending from two cylinders in the other bank have the same structure, and a communication pipe directing to a vehicle width direction communicates with right and left collecting pipes.
In the existing exhaust system for the four-cylinder internal combustion engine, in the whole area from a low rotational speed to a high rotational speed of engine speed, there is a frequent emphasis on obtaining target output characteristics. In the same way as that in JP Patent No. 4653064, countermeasures wherein the right and left collecting pipes are further partially allowed to communicate with each other or the like are taken to thereby obtain the required output characteristics by using a pulsation effect.
Recently, even in a vehicle mounted with a V-shaped four-cylinder internal combustion engine such as disclosed in JP Patent No. 4653064, there is a growing need for enjoining an exhaust sound. Technology to improve the exhaust sound has been desired.
Various kinds of internal combustion engines for a saddle-ride type vehicle have respective peculiar exhaust sound characteristics according to the types of the internal combustion engines. It is said that a favorable exhaust sound is typically different according to the types of the internal combustion engines.
For example, it is said that an exhaust sound with pulse feeling is typically favorable in a single cylinder internal combustion engine. In addition, it is said that in an inline four-cylinder internal combustion engine at a crank of 180 degrees, a high-pitched sound at high internal combustion engine speed is excellent because it is emotional.
However, in the inline four-cylinder internal combustion engine, it is necessary to achieve a high internal combustion engine speed in order to output the high-pitched sound. Depending on a rider, he/she may prefer the favorable exhaust sound in a normal area at low/middle rotational speed.
As the favorable exhaust sound at the low/middle rotational speed, an example is an exhaust sound from a large displacement V-shaped two-cylinder internal combustion engine mounted in a cruiser type motorcycle disclosed in JP-A No. 2010-174834. In the two-cylinder internal combustion engine having relatively large displacement, the peculiar thick exhaust sound having the pulse feeling is output.
On the other hand, in the two-cylinder internal combustion engine having the relatively large displacement, the exhaust sound in the normal area is favorable. However, because of properties of the internal combustion engine, output performance is not easily improved in comparison with the four-cylinder internal combustion engine having the same displacement.